Quarterly Financial Information (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			12 Months Ended		3 Months Ended		12 Months Ended	3 Months Ended			12 Months Ended	3 Months Ended				12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2012 Potomac Electric Power Co [Member]	Jun. 30, 2011 Potomac Electric Power Co [Member]	Dec. 31, 2011 Potomac Electric Power Co [Member]	Sep. 30, 2012 Delmarva Power & Light Co/De [Member]	Jun. 30, 2012 Delmarva Power & Light Co/De [Member]	Sep. 30, 2011 Delmarva Power & Light Co/De [Member]	Dec. 31, 2011 Delmarva Power & Light Co/De [Member]	Sep. 30, 2012 Atlantic City Electric Co [Member]	Jun. 30, 2012 Atlantic City Electric Co [Member]	Jun. 30, 2011 Atlantic City Electric Co [Member]	Dec. 31, 2010 Atlantic City Electric Co [Member]	Dec. 31, 2011 Atlantic City Electric Co [Member]
Quarterly Financial Information [Line Items]
Impairment losses, pre-tax				$ 12
Impairment losses, after-tax				7
Gain on early termination of finance leases held in trust	39	39		39	39
Gain on early termination of leases, after tax	9	3		9	3
Charges related to tax consequences associated with early termination of finance leases held in trust	16	22
Tax benefit/expense associated with Federal tax liabilities		14				5				4				1
Additional tax benefit or expense			17	(3)		4	5	(1)	1		1	4	3		1	1	3
Tax benefits related to uncertain tax position IRS						$ 10